Many of the questions on this list relate to Congo Red Agar, so I am hopeful
that someone will relate to my experience with it. I plan on using CRA to
demonstrate biofilm formation in staph grown under different conditions on
agar. As mentioned in Freeman and other sources, colonies should be
somewhere between pink and black, depending on biofilm formation. However,
in my experience, colonies are reliably red, while agar fades and begins do
show dark spots of pigment collection. The link below shows a plate in which
I have removed a section of membrane on which S. aureus were growing,
revealing black spots within the agar. This happens in the presence and
absence of membranes. Has anyone had an experience like this? Is it to be
expected, or should it affect my interpretation of staph colony morphology?
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